Crosslinked chitosan material capable of being used for humidity sensing

Abstract

The invention discloses a cross-linked chitosan material capable of being used for humidity sensing. The cross-linked chitosan material simultaneously has the functions of a drying agent and humidity sensing. The material is prepared by cross-linking reaction of chitosan and polybasic acid, and in a preferable scheme, the material contains inorganic salts such as calcium chloride, magnesium chloride and the like. In the preparation process, carboxyl of polybasic acid and amino of chitosan are subjected to a dehydration condensation reaction to form a stable amido bond, so that chitosan molecules are crosslinked to construct a porous structure. The structure provides a large amount of space for moisture adsorption, retains a large amount of hydrophilic groups such as hydroxyl, carboxyl and the like, and can be used as a drying agent. The material has super-long room-temperature phosphorescence, and the phosphorescence intensity and the phosphorescence service life are reduced along with the increase of water absorption, so that the material can be used as a humidity sensor to be applied to food detection. The raw materials of the cross-linked chitosan material are safe and non-toxic, and chitosan and polybasic acid are used as common additives for food, so that the safety of the cross-linked chitosan material is remarkably higher than that of an artificially synthesized additive. Other harmful chemical reagents are not introduced in the material preparation process. The material has drying and humidity indicating functions, and is suitable for food packaging.

Description

Technical Field

[0001] The present invention relates to the fields of phosphorescent materials and food packaging auxiliary materials, and particularly relates to an environment-friendly food desiccant for humidity sensing, a preparation method thereof, and an application thereof. Background Art

[0002] During the food preservation process, humidity is one of the key factors affecting food quality and shelf life. Excessive humidity will accelerate the reproduction of microorganisms (such as molds and bacteria), which will in turn lead to food spoilage. By monitoring humidity in real time, the storage environment can be adjusted in a timely manner to extend the shelf life of food. Precise humidity control can avoid the scrapping of food due to moisture absorption or drying failure, reducing economic losses and environmental impacts. Food desiccants are used to reduce the humidity inside food packaging to prevent food from mildewing and deteriorating due to moisture absorption; humidity indicators are used to monitor the humidity changes inside the packaging to provide environmental information for the storage and sale of food.

[0003] Traditional food humidity indicators such as silica gel for color change have a certain moisture absorption capacity, but silica gel is an artificial synthetic material and there is a risk of accidental ingestion by children. And common humidity indicators such as cobalt chloride test paper (Reference: Colorimetric Humidity Sensor Based on Cobalt Chloride Modified Cellulose Paper [J]. Chemical Industry Progress, 2021, 40(5): 2703-2709) have certain toxicity for the cobalt chloride in the sensor. If used in food packaging, there are safety hazards. Some chemical materials developed in recent years can be used for humidity sensing of drugs and food through color or fluorescence changes (Reference: Colorimetric Humidity Sensor Based on Liquid Composite Materials for the Monitoring of Food and Pharmaceuticals. Langmuir, 2014, 30, 10785–10791. Diphenyl imidazole-based fluorescent chemosensor for Al 3+ and its Al 3+ complex toward water detection in food products. Food Chemistry, 2023, 420, 136138.). The toxicity of these materials is not clear, which may contaminate food and there is a risk of accidental ingestion.

[0004] In addition to humidity indicators, some desiccants such as silica gel and calcium oxide are also widely used inside food packaging. Traditional desiccant calcium oxide has certain corrosiveness (References: Liu Jinhong, Zhang Fanghui. Preparation of desiccant for OLED thin films and its influence on OLED [J]. Chinese Journal of Luminescence, 2017, 38(1): 76-84. DOI: 10.3788 / fgxb20173801.0076.). Patent [CN201510255041.X] discloses a quicklime desiccant. Ethylene oxide added during the preparation process belongs to flammable and explosive substances, having certain safety hazards, and its environmental friendliness and humidity sensing function in food applications need to be improved; the food desiccant modified by chitosan quaternary ammonium salt involved in patent [CN202011359290.0] has a relatively complex composition. For example, if a large amount of magnesium sulfate is accidentally ingested, it may cause adverse reactions such as abdominal pain and diarrhea; the starch graft polymer desiccant disclosed in patent [CN202410610982.X] has a complex production process. Chemical reagents such as allyltrimethylammonium chloride, ammonium persulfate, and potassium persulfate are used during the preparation process, and its safety is not clear.

[0005] Currently, there is a lack of a food packaging auxiliary material on the market that can both dry efficiently and monitor humidity in real time, and is environmentally friendly, non-toxic, and low-cost. Therefore, developing a new type of environmentally friendly food desiccant that can be used for humidity sensing has important practical significance. Summary of the Invention

[0006] In order to solve the deficiencies of existing food desiccants and humidity indicators, the present invention discloses a crosslinked chitosan porous material, which is characterized in that: it is prepared by crosslinking reaction of chitosan with polybasic acid, the reaction solvent is water, and the water is removed by drying; the polybasic acid is selected from at least one of citric acid, oxalic acid, malic acid, tartaric acid, and malonic acid; the molar ratio of the carboxyl group of the polybasic acid to the amino group of chitosan is 0.2-2.

[0007] The above material can be used as a food desiccant. The main raw material chitosan has a wide source, such as shrimp and crab shells. The polybasic acid contains multiple carboxyl groups, and chitosan contains amino groups. Under heating conditions, the carboxyl groups of the polybasic acid and the amino groups of chitosan undergo dehydration condensation reactions to form stable amide bonds, thereby crosslinking chitosan molecules and constructing a porous structure. This structure provides a large amount of space for water adsorption, and at the same time retains a large number of hydrophilic groups such as hydroxyl groups, carboxyl groups, and amino groups, thus realizing the water absorption function and can be used as a desiccant.

[0008] The preparation method of the above material is as follows: Select chitosan and react with an aqueous solution of polybasic acid at high temperature. After cooling, dry to remove moisture to obtain the material. After the material is dehydrated, the phosphorescence is significantly enhanced, and the phosphorescence lifetime can be up to several hundred milliseconds. After excitation with a 310-370 nm ultraviolet lamp, significant long afterglow luminescence can be seen with the naked eye. The phosphorescence of these materials is mainly between 400-650 nm, and most of the phosphorescence peaks are between 450-550 nm ( Figure 1 ).

[0009] In the above preparation process, the solvent is preferably water, which is safe and harmless and does not introduce new impurities.

[0010] In order to improve the crosslinking degree of the material, the present invention discloses a preferred crosslinked chitosan porous material, which is characterized in that: the deacetylation degree of the chitosan is not less than 90%. The higher the deacetylation degree, the more crosslinkable amino groups, which can stabilize the structure of the crosslinked chitosan porous material and optimize the material properties.

[0011] In order to reduce costs and ensure the safety of the material, the above material is characterized in that: the polybasic acid is selected from citric acid and tartaric acid, which are widely sourced and low-cost, meeting the requirements of large-scale production; and are safe and non-toxic.

[0012] Since the carboxyl group and the amino group react in a 1:1 ratio to form an amide bond, an appropriate feeding ratio is required to ensure the effective crosslinking of chitosan and polybasic acid. A further preferred scheme of the above material is that: the molar ratio of the carboxyl group of the polybasic acid to the amino group of chitosan is 0.5-1.5. This ratio can enable the two to react fully, achieve efficient crosslinking, form a product with an ideal structure and pores, an appropriate number of hydrophilic groups, the best moisture absorption and phosphorescence properties, and also improve the utilization rate of raw materials.

[0013] The above-mentioned various cross-linked chitosan materials can not only be used as desiccants but also as humidity sensors. The principle of humidity sensing is as follows: The phosphorescence of chitosan decreases with the increase in water content; removing water from the chitosan material can improve the tightness of molecular packing, inhibit non-radiative relaxation by reducing molecular vibration, and enhance long-lived phosphorescence; conversely, when the water content increases, hydrogen bonds are formed with the polysaccharide chain, triggering non-radiative relaxation in the excited state and resulting in weakened luminescence (Reference: Zeng L, Zhu Z, Mo R, Li W, Xu W, Tian D. Luminescence lifetime tuning of non-conjugated organic clusters through external heavy-atom effect for smartphone-based time-resolved imaging. Chemical Engineering Journal 2023, 460, 141452.). Compared with the chitosan in this literature, the cross-linked chitosan disclosed in the present invention has significantly improved the stability of the excited state due to the formation of cross-linked covalent bonds, resulting in a significant increase in its phosphorescence lifetime and significantly improving the phosphorescence humidity tolerance of the material. Compared with other biomass materials, cross-linked chitosan can achieve phosphorescence sensing in a wider humidity range (Reference: Chen, W.; Zhu, Z. Ultralong luminescence lifetime imaging of edible plant tissue for humidity sensing in food packaging by a smartphone. Food Chemistry 2024, 454, 139778.).

[0014] To achieve better drying effects and enhance the phosphorescence of the material, the present invention discloses a preferred cross-linked chitosan porous material, which is characterized in that it is prepared by cross-linking reaction of chitosan and polybasic acid in an aqueous solution containing inorganic salts, and water is removed by drying. The polybasic acid is selected from at least one of citric acid, oxalic acid, malic acid, tartaric acid, and malonic acid, and the molar ratio of the carboxyl group of the polybasic acid to the amino group of chitosan is 0.2 - 2. The inorganic salt is selected from calcium chloride, magnesium chloride, sodium chloride, potassium chloride, and zinc chloride; the molar ratio of the inorganic salt to the amino group of chitosan is 0.01 - 1.

[0015] The role of inorganic salts is as follows: salts have strong hygroscopicity and can greatly improve the water absorption capacity of desiccants; the metal ions of salts can stabilize the excited state of chitosan through electrostatic and coordination effects, thereby enhancing phosphorescence and facilitating accurate humidity monitoring (Reference: Chemical Engineering Journal. 2023, 460, 141452). Moreover, by doping halogens, the heavy atom effect can be generated to promote intersystem crossing, produce more triplets, and result in higher phosphorescence intensity, thereby regulating the phosphorescence lifetime (Reference: Efficient Persistent Luminescence from Cellulose–Halide Mixtures for Optical Encryption. ACS Sustainable Chemistry & Engineering 2022, 10(50):16752-16759).

[0016] The preparation method of the above materials is similar to that of crosslinked chitosan without inorganic salts. Just use an appropriate amount of inorganic salt aqueous solution as the reaction solvent for crosslinking reaction, and the water is removed by methods such as distillation, vacuum drying or freeze-drying.

[0017] A further preferred scheme of the above materials is that the inorganic salt is selected from calcium chloride and magnesium chloride. As legal food additives, they have high safety and meet the requirements for the use of food desiccants (Reference: National Health Commission of the People's Republic of China. National Food Safety Standard Food Additive Calcium Chloride: GB 1886.45-2016[S]. 2016).

[0018] Through the synergistic effect of crosslinked chitosan and inorganic salts, the internal structure of the material is optimized, the luminescence stability is enhanced, and relatively stable luminescence performance can be maintained under different environmental conditions, providing a reliable signal output for humidity sensing.

[0019] The luminescence lifetime of the crosslinked chitosan porous material of the present invention is as long as 0.8 to 1 second, and the phosphorescence duration reaches more than 5 seconds, which is significantly higher than the phosphorescence of chitosan itself (Reference: Chemical Engineering Journal. 2023, 460, 141452). On the one hand, the crosslinked structure restricts the energy transfer and vibration within the molecule, reducing the probability of non-radiative transition; on the other hand, the addition of inorganic salts further stabilizes the molecular configuration and prolongs the lifetime of the excited state, thereby enhancing the luminescence lifetime and being conducive to more accurate humidity monitoring.

[0020] This desiccant uses chitosan and polybasic acid as raw materials, and forms a porous material containing a large number of hydroxyl, carboxyl, and amino groups through cross-linking reactions. These polar groups have a strong affinity for water molecules and can efficiently adsorb moisture. In the preferred embodiment, inorganic salts are added. Utilizing the strong hygroscopicity of inorganic salts, the overall water absorption capacity of the desiccant is further enhanced, enabling it to achieve a better drying effect in a shorter time.

[0021] Compared with traditional desiccants, this environmentally friendly food desiccant based on cross-linked chitosan has significant novelty and innovation. Its innovation lies in combining the humidity sensing function with the drying function. While drying the internal environment of food packaging, it can simultaneously indicate the dryness inside the food packaging in real time, providing double guarantees for ensuring the integrity of food packaging and food quality. At the same time, the raw materials are safe, cheap, and widely sourced, with good application prospects. It can be compatible with existing food packaging preparation and processing technologies, and can thus be used to prepare humidity sensing and drying integrated materials for various food packaging to meet the packaging requirements of different foods. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the phosphorescence spectrum of the chitosan cross-linked product.

[0023] Figure 2 It is the phosphorescence decay curve of the chitosan cross-linked product.

[0024] Figure 3 It is the phosphorescence lifetime imaging diagram of the chitosan cross-linked product.

[0025] Figure 4 It is the delayed luminescence imaging diagram of the chitosan cross-linked product after water absorption.

[0026] Figure 5 It is the phosphorescence decay curve of the chitosan cross-linked product containing salt. DETAILED DESCRIPTION OF THE INVENTION

[0027] Example 1 Chitosan Cross-linked Product Preparation method: Weigh 2 grams (0.01 mol) of citric acid and 1.687 grams (0.01 mol) of chitosan (degree of deacetylation 90%), dissolve them in water, stir well to dissolve, place them in an oven at 80 °C - 120 °C for reaction for 2 hours. After the reaction is completed, cool, and carry out vacuum low-temperature drying for more than 48 hours to obtain cross-linked chitosan. Take it out and excite it with a 365 nm ultraviolet LED in a dark room at room temperature. After turning off the light, a blue-green afterglow can be seen with the naked eye. The luminescence lifetime of the cross-linked material can reach 0.8 seconds ( Figure 2 ), which is significantly higher than the self-phosphorescence lifetime of 0.5 seconds of dry chitosan (Chemical Engineering Journal. 2023, 460, 141452).

[0028] Place the above materials in an environment with different humidities for more than 2 hours, then excite them with an ultraviolet lamp, take pictures of the phosphorescence change over time with a mobile phone, and through image processing, phosphorescence lifetime imaging can be obtained. The results are as Figure 3 shown. As the humidity increases, the phosphorescence lifetime decreases. Even when the relative humidity reaches 80%, there is still significant phosphorescence. The humidity response range of this material is 11% - 80%. Compared with the humidity range of 17% - 53% of garlic in the literature (Food Chemistry 2024, 454, 139778.), the humidity sensing range is wider.

[0029] Put 0.2189 g of the above materials into a high - humidity environment, and the water absorption is detected by weighing as follows:

[0030] The water absorption mass ratio of patent [CN202011359290.0] reaches 39%. The water content of silica gel is generally 20%, and the water content of montmorillonite desiccant is 20% - 30%. Compared with them, this material has higher desiccant performance, is safe and harmless, and there is also an obvious phosphorescence delay when the water content is 11.5% ( Figure 4 ).

[0031] Example 2 Cross - linked chitosan product containing salt Preparation method: Take 0.0222 g of calcium chloride (0.2 mmol), 0.0957 g (0.478 mmol) of citric acid and 0.0806 g (0.478 mmol monomer) of chitosan (degree of deacetylation 90%), add water and stir well to dissolve, put it in an oven at 80℃ - 120℃ for reaction for 2 hours. After the reaction is completed, cool it, and dry it under vacuum at low temperature for more than 48 hours to obtain the cross - linked chitosan desiccant. Take it out and excite it with a 365 - nm ultraviolet LED in a dark room at room temperature. After turning off the light, green afterglow can be seen with the naked eye.

[0032] The phosphorescence lifetime value of the product containing salt ( Figure 5 ) is greater than the self - phosphorescence lifetime of dry chitosan. The water absorption mass ratio of the product containing salt also increases. Under the same test conditions, it has stronger hygroscopicity and higher phosphorescence intensity than that without calcium chloride.

[0033] The above examples verify the humidity sensing and drying functions of the cross - linked chitosan product. By doping inorganic salts, the long - persistent luminescence and water absorption performance can be significantly enhanced.

[0034] According to a similar principle, by adjusting the raw material ratio within an appropriate range, corresponding cross - linked chitosan products can also be obtained, and all of them can be used for food drying and humidity detection.

Claims

1. A cross-linked chitosan porous material, characterized in that: The invention is prepared by cross-linking reaction of chitosan and polyacid, the reaction solvent is water, and the water is removed by drying; the polyacid is selected from at least one of citric acid, oxalic acid, malic acid, tartaric acid and malonic acid; the molar ratio of polyacid carboxyl to chitosan amino group is 0.2-2.

2. The material according to claim 1, characterized in that The deacetylation degree of chitosan is not less than 90%.

3. The material according to claim 2, characterized in that The polyacid is selected from citric acid and tartaric acid.

4. The material according to claim 3, characterized in that The molar ratio of the polyacid carboxyl group to the chitosan amino group is 0.5-1.

5.

5. A cross-linked chitosan porous material, characterized in that: The invention is prepared by cross-linking reaction of chitosan and polyacid in an aqueous solution containing inorganic salt, and water is removed by drying; the polyacid is selected from at least one of citric acid, oxalic acid, malic acid, tartaric acid and malonic acid; the molar ratio of the polyacid carboxyl group to the chitosan amino group is 0.2-2; the inorganic salt is selected from calcium chloride, magnesium chloride, sodium chloride, potassium chloride and zinc chloride; the molar ratio of the inorganic salt to the chitosan amino group is 0.01-1.

6. The material according to claim 5, characterized in that The inorganic salt is selected from calcium chloride and magnesium chloride.

7. The porous material according to any one of claims 1 to 6, used as a food desiccant and humidity sensor.

Images